By analysing the process by which host cell DNA sequences are inserted into the SV40 and polyoma virus genomes (substituted viruses), information is sought on (a) the nature of the cell genome-virus genome recombination sites, (b) the relationship of this recombination process to virus-induced neoplastic transformation, and (c) the possible activities of substituted tumor viruses of mutagens and/or as transducers of cellular genes.